The present invention relates to an image forming apparatus of an electrophotographic type such as a copying machine, a printer and a facsimile machine wherein a charging means, an image writing means and a developing means are arranged around an image carrier to transfer toner images formed on the image carrier onto a recording sheet and to fix them thereon, and in particular, to an image forming apparatus capable of forming images on both sides of a recording sheet.
In two-sided image forming, there has so far been employed a method wherein an image formed on one side of an image carrier is transferred and fixed on a recording sheet which is then kept temporarily in an intermediate tray, and the recording sheet is fed out of the intermediate tray in synchronization with an image formed next on the image carrier so that the image is transferred and fixed on the other side of the recording sheet.
In the two-sided image forming apparatus of this type, conveyance operations for a recording sheet such as conveyance thereof to an intermediate tray and conveying a recording sheet through a fixing unit two times are conducted as stated above. Therefore, reliability for conveyance of a recording sheet is low, resulting in causes for jamming and creases on the recording sheet.
For the situation mentioned above, TOKKOSHO Nos. 49-37538 and 54-28740 and TOKKAIHEI Nos. 1-44457 and 4-214576 are suggesting an image forming apparatus and an image forming method wherein a first image carrying means and a second image carrying means are used so that toner images are formed on both sides of a recording sheet and then are fixed at the same time.
Further, the inventors of the invention are studying an image forming apparatus and an image forming method wherein a plurality of toner image forming means each being composed of a charging means, an image writing means and a developing means are arranged around a photoreceptor drum (first image carrying means), and color toner images formed and superposed on the photoreceptor drum are collectively transferred onto a toner image receiving body (second image carrying means) temporarily, and then, superposed color toner images are formed on the photoreceptor drum next, thus, both superposed color toner images are transferred respectively on both sides of a recording sheet supplied in synchronization with the toner images on the photoreceptor drum and the toner images on the toner image receiving body, with the toner images on the photoreceptor drum representing an image for obverse side and the toner images on the toner image receiving body representing an image for reverse side, after that, the recording sheet is separated from the toner image receiving body and the toner images on the recording sheet are fixed, so that two-sided color images are formed.
In the image forming apparatus and the image forming method, the toner image receiving body (second image carrying means) and the fixing unit are arranged to be close to each other so that the recording sheet separated from the toner image receiving body may be fed into the fixing unit directly, because the recording sheet having on its both sides unfixed toner images needs to be conveyed to the fixing unit (fixing means). However, when the toner image receiving body is arranged to be close to the fixing unit, there are caused problems that the toner image receiving body is deformed by heat coming from the fixing unit, toner images transferred onto the toner image receiving body are slightly fused to become difficult to be transferred, or toner sticks to the toner image receiving body. In addition, there are caused problems that toner images on the recording sheet are disturbed, or creases are caused on the recording sheet when the recording sheet is held in the fixing unit to be sandwiched therein, resulting in inability of forming excellent two-sided images, because conveyance of the recording sheet separated from the toner image receiving body is not conveyed uniformly but is conveyed ruggedly depending on the type of the recording sheet.